1. Field of the Invention
The present invention relates to a semiconductor device having bumps for connection between a semiconductor chip and an external electrode.
2. Related Background Art
The wireless bonding is a method for connecting all pads on a semiconductor chip to external terminals by respectively associated bumps, which includes the flip chip method and the TAB (Tape Automated Bonding) method.
FIG. 1 is a cross section to show a structure of a conventional semiconductor chip on which a Au (gold) bump is formed. The conventional semiconductor chip is formed by depositing a Ti (titanium)-W (tungsten) alloy 32 on an Al (aluminum) pad 31, a Au layer 33 on the Ti--W alloy 32, and a Au bump 34 on the Au layer 33 by electroplating. A bump forming process is described for example in Reference "Semiconductor Fabrication Technology Handbook" (handoutai-jissou-gijyutsu Handbook), Chapter 2, Section 9, p.128-138, Flip Chip Bonding, Author. Soga Fusao, Sept., 1986.
Incidentally, the mounting (packaging) method using the Au--Sn (gold-tin) eutectic method is such that Sn is deposited by plating on a pad on an external electrode and the Au--Sn eutectic alloy reaction is undergone between the pad and an associated Au bump on semiconductor chip, whereby a surface portion of Au bump melts to effect connection therebetween.
The Au--Sn eutectic alloy reaction, however, takes place only at a contact surface between the Au bump and the Sn plating layer on the pad. Accordingly, if Au bumps on a semiconductor chip have a disparity in height thereof, there would remain some Au bumps not contacting with corresponding pads, resulting in defective contact therebetween. FIG. 2 shows a state of an Au bump in defective contact with an opposing pad. FIG. 2 shows an example in which a semiconductor chip 42 is mounted on a wiring substrate 41. Since a bump 44 is not in contact with a Sn plating 47 on a pad 46, no Au--Sn eutectic alloy reaction takes place at the surface of the bump 44, whereby resulting in defective contact therebetween.